1. Field of the Invention
The present invention relates to a heat-sealing method and a heat-sealing apparatus for fusing a fiber sheet of a bulky nonwoven fabric, a fiber web, a composite of a nonwoven fabric and a fiber web, a composite of a film and a fiber web, a composite of a film, a nonwoven fabric and a fiber web, or the like into a predetermined pattern.
2. Description of the Prior Art
A fiber sheet of a nonwoven fabric, a fiber web or the like may be partially fused and used as a cleaning sheet. In Unexamined Published Japanese Patent Application No. 11-235301, for example, there is disclosed a cleaning sheet which is manufactured by heat-sealing a fiber sheet, as prepared by overlapping a fiber web of long fibers opened from TOW on a nonwoven fabric, partially and by slitting the nonwoven fabric and the fiber web in the region between fused lines formed by heat-sealing. In this cleaning sheet, the fiber web can sweep dust easily with brush-shaped portions, each of which is located between one fused line and one slit, and the fiber web is effective to trap the dust or relatively large pieces of trash with remaining portions, each of which is located between two fused lines while having no slit therebetween.
The heat-sealing method of the prior art for partially fusing a relatively bulky fiber sheet such as a laminate of the nonwoven fabric and the fiber web is performed by using rolls, as shown in FIG. 7.
In the heat-sealing method shown in FIG. 7, a pair of rolls 31 and 32 are provided integrally with stealing ridges 31a and 32a of a predetermined pattern on their respective outer circumferences. These rolls 31 and 32 are heated to a temperature higher than the melting point of the fibers composing a fiber sheet 10 and are rotated in the directions of arrows.
The fiber sheet 10 is prepared by laminating a fiber web 12 of long fibers opened from TOW on a base material sheet 11 of a nonwoven fabric and/or a film. The fibers forming the base material sheet 11 and the fiber web 12 contain fusible fibers such as those of PET, PE or PP.
As the fiber sheet 10 is fed into the clearance between the rotating rolls 31 and 32, the fiber sheet is partially pressed and heated between the sealing ridges 31a and 32a of the rotating rolls 31 and 32 so that the fusible fibers are fused to form fused portions 13. These fused portions 13 are formed into a pattern identical to the pattern of the sealing ridges 31a and 32a. 
The heat-sealing method using the rolls 31 and 32, as shown in FIG. 7, can perform the high-speed treatment when it is to fuse a less bulky nonwoven fabric or the like but is defective in that it has a poor fusing efficiency for a relatively bulky sheet such as the fiber sheet 10 including the fiber web 12 opened from TOW and is difficult to perform the high-speed treatment.
Specifically, the fibers, as opened from TOW, are in a crimped state so that the fiber web 12 has a low density and a high volume of voids. When this bulky fiber sheet 10 is fed into the clearance between the rolls 31 and 32 so that it is pressed between the sealing ridges 31a and 32a, the presence of the air in the voids in the fiber sheet 10 deteriorates the thermal conductivity in the fiber sheet 10 so that the fusible fibers take a time to be heated to a temperature higher than their melting point. Before the fiber sheet 10 is completely clamped, as shown in FIG. 7, by the sealing ridges 31a and 32a, more specifically, the heated air in the fiber sheet 10 will go out to deteriorate the efficiency for heating the fusible fibers to the melting point or higher.
In order to fuse the fusible fibers to form the fused portions 13, therefore, the speed of rotation of the rolls 31 and 32 has to be set at a low level so that the high-speed treatment cannot be made.
In order to eliminate this defect, it is conceivable to raise the temperature of the rolls 31 and 32 and to set the pressing forces of the rolls 31 and 32. With these settings, however, when the fiber sheet 10 is heat-sealed by the sealing ridges 31a and 32a, the fusible fibers in the fused portions 13 especially on the sealing surface side are fused out or are cut by the pressure so that the fused portions 13 easily become defective.
In the heat-sealing method using the rolls 31 and 32 shown in FIG. 7, on the other hand, the fiber sheet 10 is locally clamped between the sealing ridges 31a and 32a which are protruded from the two rolls 31 and 32. When the fiber sheet 10 has portions of different thicknesses or when the pattern of the sealing ridges 31a and 32a is formed, for example, into a V-shape or a curved shape, therefore, the fiber sheet 10 is wrinkled or offset, as clamped by the sealing ridges 31a and 32a, or the fiber sheet 10 or its fibers themselves, as opened from TOW, are meandered so that the fused portions 13 may be unable to have a precise pattern.
The invention has an object to provide a heat-sealing method and a heat-sealing apparatus for forming fused portions at a high speed even in a relatively bulky fiber sheet.
Another object of the invention is to provide a heat-sealing method and a heat-sealing apparatus for forming fused lines in a precise pattern, when the fiber sheet has portions of different thicknesses or when the fused portions have a V-shaped or curved pattern, by preventing the fiber sheet from being wrinkled or offset and the fiber sheet and its fibers themselves from being meandered.
According to an aspect of the invention, there is provided a heat-sealing method comprising: feeding a fusible fiber sheet into the clearance between a pair of rolls, at least one of which has sealing ridges of a predetermined pattern on its outer circumference; and heating and pressing the fiber sheet with the sealing ridges, to form fused portions corresponding to the pattern of the sealing ridges, in the fiber sheet,
wherein an insulator is disposed to cover the outer circumference of the roll, as lacking the sealing ridges, and is made of a material which has a lower thermal conductivity than that of the sealing ridges and which can be elastically contracted by the pressure of the rolls; and
wherein the fiber sheet to be fed into the clearance between the rolls rotating is compressed by the insulator to discharge its internal air and is then heated and pressed by the sealing ridges so that it is heat-sealed.
When the rolls are rotated and fed inbetween with a bulky fiber sheet, according to the invention, the fiber sheet is clamped by the insulator to expel the air from the fiber sheet to the outside, so that it is clamped by the sealing ridges when the air is expelled to a considerable extent. At the instant when the fiber sheet is clamped by the sealing ridges, therefore, the void volume in the fiber sheet is reduced so that the fusible fibers in the fiber sheet are quickly heated by the heat of the sealing ridges. Therefore, the reliable heat-sealing can be ensured even if the rotating speed of the rolls is increased to raise the feeding rate of the fiber sheet.
On the other hand, since the bulky fiber sheet, as formed of a fibrous layer, is clamped by the sealing ridges after it was crushed by the insulators to some extent, even if the fiber sheet has the different thicknesses or if the fused portions have the V-shaped or curved pattern, the fiber sheet is neither wrinkled or offset, nor meandered are the fiber sheet or the fibers themselves, so that the fused portions formed can have a precise pattern without going out of position.
According to another aspect of the invention to be employed in the aforementioned heat-sealing method, there is provided a heat-sealing apparatus comprising a pair of rolls for heating and pressing a fusible fiber sheet to form fused portions of a predetermined pattern,
wherein at least one of the paired rolls is provided with sealing ridges on its outer circumference for forming the fused portions of the predetermined pattern and with an insulator covering the region other than the sealing ridges; and
wherein the insulator is made of a material which has a lower thermal conductivity than that of the sealing ridges and which can be elastically contracted by the pressure of the rolls.
In the heat-sealing method and the heat-sealing apparatus, the insulator preferably has a thickness, when unexposed to the pressure of the rolls, equal to or larger than the height of the sealing ridges. Moreover, the insulator preferably has a thickness, when exposed to the pressure of the rolls being pressed, equal to or smaller than the height of the sealing ridges.